1. Field of the Invention
The invention pertains generally to position sensing apparatus and is more particularly directed to a signal conditioning circuit for a rotational magnetic position sensor.
2. Description of the Prior Art
Rotational magnetic position sensors or reluctance sensors are conventional in the art. Generally, such sensors have a rotating member of a highly permissible material. Magnetically coupled to the rotating member is a sensor pickup for determining the angular relation of the member to the pickup. At predetermined angular events the rotating member may have various means for changing the magnetic coupling between the sensor pickup and the rotating member such as cogs, slots, or holes. As each event passes the sensor pickup, an output is developed which has a positive and negative peak with a zero crossing centered substantially with the time alignment of the sensor and event.
Magnetic sensors such as these are advantageously used in many applications. One of these utilizations that will increase in importance in the future is the position sensing of an internal combustion engine crankshaft, camshaft, or pully wheel attached thereto. With rotational information from the sensor-speed, ignition timing, fuel injection cycles, etc. can be accurately determined. This is accomplished by positioning the angular events on the rotating member in a predetermined relationship with an engine event such as a mark for TDC for the number one cylinder and other marks for fixed increments of degrees around the revolution of the crank shaft.
There is, however, a problem sensing the angular events when magnetic sensors are used in internal combustion engine applications. The operating range of RPM under normal conditions for an engine may vary from 30 RPM while cranking during starting conditions to over 6000 RPM while at a high speed cruise. The sensor must then reliably provide information over this large range. Many magnetic sensors provide a signal that increases with RPM as the signal amplitude is directly proportional to rate of change of magnetic flux coupling the rotating member to the magnetic sensor. The noise component of the signal due to surface imperfections, vibration, non concentric alignment, etc., also increases likewise and remains substantially at a constant percentage of the total output.
Thus, the high speed noise component may be greater than the low speed signal component making it difficult to discriminate between the two. Moreover, the signal component may vary from a few tenths of a volt to tens of volts over the RPM range complicating the discrimination effort even more. Therefore, to provide a useable output signal for processing by other engine circuitry, the sensor output must be conditioned to reject the noise and provide an indication of the angular event with accuracy.
A U.S. Pat. No. 3,801,830 issued to Boyer attempts to solve these problems by providing a comparator circuit with a variable threshold adjustment responsive to the magnitude of the immediately preceeding input signal. When a positive transition of the sensor signal exceeds this threshold the threshold is reduced to a non signal value to detect the zero crossing of the sensor signal. The system then includes a pulse shaper which generates a pulse of a fixed width in response to the positive to negative zero crossing that it has detected. This system is unduly complicated and is more expensive than necessary because it first discriminates between the noise and signal levels and then secondly senses an event, i.e., positive to negative zero crossing. A substantial reduction in circuitry and cost can be achieved by performing these functions simultaneously.